The invention relates to a mechanically cleanable screen.
Such a screen is used particularly when cooking and circulation liquids are being removed from a pressure vessel during the production of chemical pulp or paper pulp by a continuous method. The screen is primarily intended to be used for separating liquid from chips and/or pulp without the screen getting blocked and without disturbing the plug flow and other flows.
Chemically produced pulp should have both good strength properties and low kappa number, in other words low lignin content after cooking. These requirements are contradictory if the cooking conditions are not optimal. The cooking conditions include e.g. a correct alkaline distribution, suitable temperature profile, sufficient amount of liquid and as small gradients as possible particularly in the radial direction of the continuous digester. These conditions require high circulation and extraction flows. Since even partial blocking of extraction and circulation screens causes channelling and disturbs flows in the digester, and thus often results in losses in production, it is not always possible to maintain the conditions needed for producing clean pulp with good strength properties. Naturally this is highly disadvantageous since it for example increases the use of raw materials, energy and chemicals, which in turn increases the production costs and burdens the environment.
Several different solutions have been suggested for this blocking problem of the extraction and circulation screens of the continuous digester. In WO/11565 Kamyr Ab have suggested the use of several small screen elements provided with bars, which can be back-flushed with a high flow, also known as back spooling. The problem associated with this arrangement is the large amount of liquid required by back spooling, which decreases the efficiency of the flows through the screens. If the screen is seriously blocked e.g. by chips, shives and precipitates, the fluid pressure of back spooling does not necessarily provide sufficiently efficient cleaning.
Extraction and circulation screens have been conventionally arranged parallel with the pressure cover and thus also parallel with the plug flows. When the flow of cooking chemicals through the screens sucks chips, shives and pre-cooked pulp against the screens, this and the radial component of the pressure caused by the chips pillar generate a resultant force which wedges the above-mentioned particles into the slots of the screens. These particles pile up one on the top of the other from below upwards causing blockages, which disturb the plug flow and cause channelling of the flows. Kamyr Ab have tried to solve this problem in Finnish Pat. No. 54509 where a screen comprises evenly arranged vertical screen rods which define screen slots between them. Blocking is prevented by moving the bottom end of every other screen rod, the top end of which is provided with a hinge, 5 to 15 mm from its middle position to both directions, assuming that this motion will remove a wedged blockage formed by chips or pulp from a screen slot. Since the static friction between the static screen rod and the blockage is greater than the dynamic friction between the dynamic screen rod and the blockage, the motion of the hinged screen rod against the flow does not generate a component of force which would push the blockage against the flow back into the plug flow. When the moving screen rod moves back in the flow direction, the dynamic friction increases the component of force that pushes the blockage into the screen slot, the force component being produced by the radial component of the dynamic pressure of the flow and the radial component of the static pressure of the plug flow. The motion of every other screen rod cannot generate any mechanical shear force or push force which would act directly on the screen slot and clean it. Furthermore, the screen rod moving like a pendulum moves chips or pulp back and forth, simultaneously working and crushing them. This is disadvantageous since the flow carries the diminished particles through the screen into the liquid circulation. In other words, the separating capacity of the screen decreases. Thus the solution disclosed in Fl 54509 impairs the function of the screen and screen slots that are blocked e.g. by chips or pulp cannot be cleaned with it.